The invention relates to liquid jet cutting apparatus and, in particular, to a nozzle member for use in the formation of high velocity liquid jets in such apparatus.
For liquid jet cutting applications, very fine liquid jets are required and various materials have been used to produce nozzle members through which liquid at high pressure is forced. Materials used for these nozzle members have included metals, ceramic materials such as tungsten carbide, diamond and artificial jewels such as artificial sapphires. One of the most successful materials for use as a nozzle member, at pressures of up to 60,000 psi, has been sapphire which combines the desirable properties of strength and smooth surface finish. Artificial sapphire is available in the form of watch and instrument jewels, and some of these components are suitable, or can be modified, for use as nozzle members to produce high pressure liquid jets.
Thus, very fine jets of liquid, at pressures of up to 60,000 psi, are formed by passing the liquids through a nozzle member comprising a piece of artificial sapphire formed with a circular aperture having a diameter which is determined by the requirements of the material to be cut and is typically within the range of 0.05 mm to 0.40 mm.
The normal requirements for precision cutting with high pressure liquid jets are:
1. Good finish to cut edges; PA1 2. Sufficient depth of good quality cut; and PA1 3. Narrow cut. To fulfill these requirements, the energy contained in the jet must be concentrated in as small an area of the material being cut as possible. The jet must therefore be stable and non-oscillatory, with a minimum of expansion along its exposed length and it should not break up into spray or droplets. Surface roughness of any region of the internal surface of the aperture through the nozzle member which is exposed to high velocity flow of liquid is an important factor in determining the level of disturbance present in the jet as it leaves the nozzle member and, therefore, in determining the performance of the jet cutting apparatus.
It has been found that in the typical range of aperture diameters in nozzle members used for precision jet cutting, a high degree of polish of the nozzle member surfaces is difficult to achieve with many materials. With some aperture shapes, polishing is complicated by the relative inaccessibility of some of the internal surfaces. As poor surface finish can also result in local cavitation in the liquid flowing through the aperture in the nozzle member and this can further damage the internal surface of the aperture, jet performance is unreliable.
One solution, that has been applied with materials which can be polished to a sufficient degree, is to produce the liquid jet by passing the liquid through an orifice member having an orifice which prevents the liquid from contacting the inaccessible internal surface of the orifice by causing the liquid to separate from the internal surface of the orifice at a sharp junction formed between the inlet portion of the internal surface and the inaccessible surface. The disadvantage of this method is that the "quality" of the jet produced is extremely dependent on the surface finish at this junction, which is highly stressed in operation and therefore liable to sudden failure.